Would just about every car have better aero backwards? The front ends of most cars in the past couple decades seem to have the gradual taper associated with most streamlined designs I've seen, while the rears have much more abrupt leading edge seen. Kinda like the top and bottom of a teardrop, except for cars it's backwards. Especially for hatches. :p

It would be interesting to see what it looked like in a wind tunnel with smoke streams.
Wunder if anyone has done it ?

tomauto

09-16-2006 08:50 AM

I think the frontal area would be a little much. If you look at the topside of a Insight, it does resemble a teardrop.

omgwtfbyobbq

09-16-2006 08:59 AM

I think the frontal area is the same in both directions since the same area's perpendicular to the direction of motion forward or backward. Although I could be wrong....

Quote:

Originally Posted by wikipedia

The reference area A is related to, but not exactly equal to, the area of the projection of the object on a plane perpendicular to the direction of motion (ie cross-sectional area). Sometimes different reference areas are given for the same object in which case a drag coefficient corresponding to each of these different areas must be given. The reference for a wing would be the plane area rather than the frontal area.

It seems like A is matched to Cd just a bit, so there could be small variations in A backwards?

tomauto

09-16-2006 09:14 AM

Well, you want to minimize the impact of air on the car, so wedge shape makes more sense.

AlexK

09-16-2006 06:19 PM

You are absolutely right about cars not being aerodynamic. People buy cars because they look good. Just look at a 747. It is pretty blunt in the front and gradually tapered in the rear, like your streamlined shape example. Airlines don't care what a plane looks like, they want efficiency.

I'm not an aerodynamics expert but I did take a graduate level aerodynamics class while in college. The idea of streamlining is to accelerate the air around the body, and slow it back down again at the rear. When the air is slowed, its pressure increases. High pressure at the rear of the vehicle is a good thing. You don't want flow separation because when the flow separates from the object, you don't get pressure recovery beyond that point and are left with a large low pressure zone. The low pressure zone "sucks" the vehicle backward (drag). Keeping flow attached at the front is easy. That's because the flow is accelerating from a higher pressure area to lower pressure area. Keeping flow attached at the rear is tricky and pretty much impossible to keep fully attached. That's because the flow is moving from low pressure to a higher pressure (kind of like water flowing uphill... it will do it but not gracefully). If the surface is too steeply angled or changes direction too quickly, the flow will separate. The energy of the boundary layer will be shed in large vorticies and dissipated as heat instead of pressure on the rear of the vehicle.

No matter how streamlined a shape is, separation is going to happen somewhere before the trailing edge. So if the shape is cut off at that point, aerodynamics won't be affected. Many cars probably separate flow at the rear window, so what the body looks like after that won't matter too much. If the car is well designed and can keep flow attached further rearward, the shape of the trunk area will be important.

omgwtfbyobbq

09-16-2006 07:27 PM

What's a wedged shape? Google image returns this...http://www.entermyworld.com/cat/myd/...xter2kc1x1.jpg
Could we conclude that the designers couldn't keep flow attached past the point where they cut it off, or just cut it off at that point because having a boattail would look too weird?